energy ball/sludge bomb-Goes well with higher spATK. I wouldn't use petal dance since it locks him into that move, allowing your opponent to switch into a different pokemon, one more resistant and ready to deal with your confused venusaur. sludge bomb has higher power, but isn't as useful since your opponent would be poisoned by toxic.

ingrain/ synthesis-ingrain if you want even more health at the cost of switching, synthesis for instant healing

it also works well to give him the item leftovers for more healing,

or you can make him a sub seeder:

leech seed
substitute
toxic
energy ball

last, you could take the unexpected route by using swords dance ti power up venusaur, then using physical moves like double edge and earthquake to take out your foes, while healing with leech seed. The only downside to this is his low speed. I've seen this before, it can be deadly when unexpected, many foes use taunt on venusaur, not knowing that it plans on attacking them directly. He has sleep powder to, letting you stop your foes while you power up.

This guy is normally showing up with 2 or 3 STAB Grass moves, or he's mixed, which isn't THAT bad.
Curse lets him become a total tank, it's hard to do anything without being super effective, And now you can Leech Seed for Seedover combo.
Earthquake gets sweet type coverage, go figure, And Sludge bomb is his Physical STAB Option.
Decent Tank.

This thing loves the sun. Sleep powder is just amazing, as it lets Venasaur set up a Growth/Swords Dance and then decide to risk another or proceed to sweep.Seed Bomb is just there if you think that Venasaur can take a hit, or is on the field against a Hazard setter, or something.Then, use it on Kyogre, Groudon, Tyranitar, and Hippodown. Outrage is just the perfect move to hit all those Dragon types the lurk in Ubers. Earthquake it to finish off the steel types that resist Outrage. This dude is just amazing. Sadly, there are flaws.

Air Balloon Heatran and Skarmory can easily resist both attacking moves and kill Venasaur off with a STAB move.

In the end, who uses Skarmory in Ubers? Lol, beware and have fun using this set.

Its a risky set. You will need too many turns to set this babe up. Then a single Whirlwind is all you need to trash all those turns. Even a switch to the oh so common Heatran/Infernape will demolish this set.

Okay, this guy should be used along with Sunny Day, which is why you have Chlorophyll, HP Fire, and Solarbeam. Sleep Powder is helpful for facing down Fire types, which you can then 2HKO usually with your STAB Sludge Bomb. Sludge Bomb also deals with any Grass types better than HP Fire (which is more for Steel types) such as Serperior, or any Grass types on another Sun team. The EVs give it enough Speed with Chlorophyll, power, and some bulk. Leftovers helps reduce the impact of priority and residual damage.

If you do not have Sunny Day support, use Overgrow. Also, use the first set of EVs. If you do have Sunny Day support, use Chlorophyll and the second set of EVs. Curse should only be used on the Overgrow set. However, even on the Overgrow set Swords Dance is probably a better option. On the Chlorophyll set, you can use either Swords Dance or Growth. Swords Dance will make Psych Up less effective if used by a Special Attacker on Venusaur. However, Growth can help your own Psych Up user asnd more importantly it makes the foe unsure as to whether you are using a physical, special, or mixed sweeper. You need a stat boosting attack, otherwise Venusaur will not be doing anything. Razor Leaf can hit multiple foes in a multi-battle. However, its low base power might make you want to use Power Whip. Earthquake is an incredible move in multi-battles, and should be used on any Physical attacker that can learn it. Frustration is neutral type coverage.

Mega Venusaur

A tank/staller Mega Venusuar set. I would start off with Toxic, then use Amnesia to increase my special defense or stall using Protect. After that, you could heal yourself using Leech Seed / Synthesis or hit them real hard with Petal Dance / Venoshock.

Just to let you know, Speed. I'm probably going to be editing a lot of your earlier sets with revised sprites since all Pokemonstadium sprites seem to not be showing up anymore.
( PS: I'll be editing them because I'm searching out old moveset Q's for terrible " need to be hidden " movesets. If you want to call a specific region and help me do a quick search to clean things up, just umm.. comment back? Or find me in the chat. )

Sun-sweeper

Growth boosts S.Attack(And Attack but not important here) by 2 stages, making Ivysaur very strong. Solar-beam is STAB, but you could go with Giga-drain if you want healing instead of power. Sludge-bomb is STAB, but if you are running a team with Toxic-spikes then Venoshock. HP(Fire) is Coverage and is boosted by Sun.

Sun tank.
Out classed as a sweeper, considering the lowest tier he can be used in is BL.
So tanking is the way to go.
All defensive EVs and evoilite means that he can set up to +6 Sp Atk in peace.
While having Synthesis to heal off anything taken, just to start Solar + HP Fire killing.
He still gets like 300+ speed, so he's good there too, bulky sweeper, kinda.
But mostly used as a tank ( Speed used for setting up and fast recovery ).

Swords Dance gives a big boost to your attack, so you can actually start doing some damage.
Razor Leaf and Earthquake hit both opponents, while Power Whip is for focusing in on just one opponent. Miracle Seed due to double Grass STAB.